I believe you can't really use GCP without providing the billing details. However, your billing details are used solely for verification (against bots) during your trial period.
You won't be charged after your trial unless you upgrade to the paid plan. If you use the 300$ credit or the trial ends after 12 months, all running services will be stopped and your data will be available for 30 more days (except those stored on Compute Engine).
If you don't want to add your credit card, there is also an option to provide bank account information. You can see it here under No Credit Card?.
See this page for more info:
https://cloud.google.com/free/docs/gcp-free-tier